His 1887 dissertation at the University of Strassburg was on the theory of linear substitutions, known today as matrix groups.
2.
The combination of wider and wider weak, linear diffusive steps like a Hill cipher, with non-linear substitution steps, ultimately leads to a substitution-permutation network ( e . g . a Feistel cipher ), so it is possible from this extreme perspective to consider modern block ciphers as a type of polygraphic substitution.
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